Your best source of information is the Cisco Press book "Cisco Voice Gateways and Gatekeepers (GWGK)" which explains these 2 in more detail. I've included the information to help..

The Role of Voice Gateways:

In a VoIP environment, voice gateways are the interface between a VoIP network and the public switched telephone network (PSTN), a private branch exchange (PBX), or analog devices such as fax machines. In its simplest form, a voice gateway has an IP interface and a legacy telephone interface, and it handles the many tasks involved in translating between transmission formats and protocols.

At least one gateway is an essential part of any IP telephony network that interacts with the PSTN or with analog devices. In addition, when gateways are properly configured, many can take over for a Cisco CallManager when it is unreachable.

The gateway allows communication between the two networks by performing tasks such as these:

Performing call setup and teardown for calls between the VoIP and PSTN networks by terminating and reoriginating the call media and signaling.

Providing supplementary services, such as call hold and transfer.

Relaying dual tone multifrequency (DTMF) tones.

Supporting analog fax and modems over the IP network.

In a Cisco CallManager network, a gateway also needs to do the following:

- Support CallManager redundancy by rehoming to alternate CallManagers.

- Support call survivability when no CallManager is available.

Gateways communicate with other gateways, gatekeepers, their endpoints, or their call control agents, such as Cisco CallManager or a PBX.

The following are the protocols that Cisco gateways use for voice signaling and media:

Media Gateway Control Protocol (MGCP)

H.323

Session Initiation Protocol (SIP)

Skinny Client Control Protocol (SCCP)

Real-Time Transport Protocol (RTP)

The Role of Voice Gatekeepers:

Gatekeepers help VoIP networks scale to large sizes. Companies that have geographically dispersed voice networks, or networks that have become so large that they are unwieldy, might opt to segment their network. In a CallManager network, you can create multiple clusters. In that case, you would need to configure a full mesh of connections over the IP WAN to link all the segments or clusters. You would need to configure dial information for every remote location on every gateway and CallManager cluster.

A better alternative is to use gatekeepers. In a network that has gatekeepers, trunks are needed only to the gatekeeper, and the gatekeeper maintains remote endpoint information.

When you use gatekeepers, gateways and CallManagers register with their gatekeeper. Gatekeepers divide the network into "zones," or groups of devices that register with a particular gatekeeper.

When an H.323 gateway receives a call that is destined to a remote phone, it queries the gatekeeper for the location of the endpoint.

If the call is destined for a different zone, you can configure the gatekeeper to allow it only if sufficient bandwidth is available. In more complex networks, you can use a Directory gatekeeper to maintain information about all the zones.

You can configure Cisco routers with the appropriate Cisco IOS as H.323 gatekeepers.

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Your best source of information is the Cisco Press book "Cisco Voice Gateways and Gatekeepers (GWGK)" which explains these 2 in more detail. I've included the information to help..

The Role of Voice Gateways:

In a VoIP environment, voice gateways are the interface between a VoIP network and the public switched telephone network (PSTN), a private branch exchange (PBX), or analog devices such as fax machines. In its simplest form, a voice gateway has an IP interface and a legacy telephone interface, and it handles the many tasks involved in translating between transmission formats and protocols.

At least one gateway is an essential part of any IP telephony network that interacts with the PSTN or with analog devices. In addition, when gateways are properly configured, many can take over for a Cisco CallManager when it is unreachable.

The gateway allows communication between the two networks by performing tasks such as these:

Performing call setup and teardown for calls between the VoIP and PSTN networks by terminating and reoriginating the call media and signaling.

Providing supplementary services, such as call hold and transfer.

Relaying dual tone multifrequency (DTMF) tones.

Supporting analog fax and modems over the IP network.

In a Cisco CallManager network, a gateway also needs to do the following:

- Support CallManager redundancy by rehoming to alternate CallManagers.

- Support call survivability when no CallManager is available.

Gateways communicate with other gateways, gatekeepers, their endpoints, or their call control agents, such as Cisco CallManager or a PBX.

The following are the protocols that Cisco gateways use for voice signaling and media:

Media Gateway Control Protocol (MGCP)

H.323

Session Initiation Protocol (SIP)

Skinny Client Control Protocol (SCCP)

Real-Time Transport Protocol (RTP)

The Role of Voice Gatekeepers:

Gatekeepers help VoIP networks scale to large sizes. Companies that have geographically dispersed voice networks, or networks that have become so large that they are unwieldy, might opt to segment their network. In a CallManager network, you can create multiple clusters. In that case, you would need to configure a full mesh of connections over the IP WAN to link all the segments or clusters. You would need to configure dial information for every remote location on every gateway and CallManager cluster.

A better alternative is to use gatekeepers. In a network that has gatekeepers, trunks are needed only to the gatekeeper, and the gatekeeper maintains remote endpoint information.

When you use gatekeepers, gateways and CallManagers register with their gatekeeper. Gatekeepers divide the network into "zones," or groups of devices that register with a particular gatekeeper.

When an H.323 gateway receives a call that is destined to a remote phone, it queries the gatekeeper for the location of the endpoint.

If the call is destined for a different zone, you can configure the gatekeeper to allow it only if sufficient bandwidth is available. In more complex networks, you can use a Directory gatekeeper to maintain information about all the zones.

You can configure Cisco routers with the appropriate Cisco IOS as H.323 gatekeepers.

Basically, gateways facilitate comms between POTS and VoIP - it's an entry point which converts one type of traffic into another (analog\digital to IP). Think of it as a translator..

The gatekeeper (H323) is an optional component (can be hardware or software), however when it's included it becomes the central administrative point. Think of it as it's own entity which maintains remote endpoint information for it's own area. A trunk link then connects it back to the CallManager.

I have LG ARIA exchanges with analogue phones which in turn is connected on E1 port on CISCO 7206 VXR router at 17 locations. These 17 locations also has IP phones which are registered on call manager 5.1. I have a voice gateway with 1 PVDM2-64 and one PVDM2-32. These 17 locations get connected on a central location where the call manager is located. How do I make my IP phones and analogue phones communicate with each other ?